US7511695B2 - Display unit and backlight unit - Google Patents

Abstract

The screen is exempt from variation in brightness due to uneven temperature distribution.

The color liquid crystal display unit has a color display panel of transmissive type and a backlight unit placed behind the color display panel. The backlight unit has a plurality of LEDs connected in series, the drive unit to drive the LEDs in correspondence with the LEDs, and the temperature sensor to detect the temperature of LEDs. The groups of LEDs are arranged in regions where the display unit has the same temperature. The drive unit controls current to be supplied to LEDs in response to temperature detected by the temperature sensor so that the LEDs retain the constant brightness even when the LEDs fluctuate in temperature.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a display unit with a non-luminous screen of transmissive-type and also to a backlight unit placed behind the screen.

Liquid crystal panels are usually provided with a backlight unit composed of cold cathode fluorescent lamps (CCFL). However, there is a demand for a mercury-free backlight unit from the standpoint of environmental protection. Light emitting diodes (LEDs) are regarded as promising in this respect as a substitute for CCFLs. Application of LEDs to televisions is under meticulous research because of their ability to produce a well-balanced white color by optical color mixing from LEDs emitting red, green, and blue elementary colors.

The disadvantage of using LEDs as the light source is the necessity of supplying red LEDs, green LEDs, and blue LEDs with current independently because they differ in luminous efficiency. Moreover, LEDs vary in their semiconductor compositions depending on their emitting color and hence vary in their driving voltage and power consumption. However, it is not practical to individually drive LEDs constituting the backlight unit.

In practical use of LEDs as the light source of backlight unit, it is common practice to divide LEDs into a certain number of groups and drive each group (in which LEDs are connected in series) as a whole.

In other words, each group consists of a prescribed number of red, green, and blue LEDs connected in series, and each group is connected to a DC-DC converter power source and a PWM control unit. Thus it is possible to adjust the tint and brightness of backlight (which result from combination of red, green, and blue) by adjusting the current (through PWM control) supplied to the LEDs in each group.

Patent Document 1:

Japanese Patent Laid-open No. 2001-272938

SUMMARY OF THE INVENTION

Incidentally, LEDs vary in amount of light emission depending on temperature and this temperature dependence also varies depending on the color they produce.FIG. 15 is a graph showing the fundamental temperature characteristics of red, green, and blue LEDs. InFIG. 15, X-axis represents the element temperature and Y-axis represents the relative brightness (or output). The relative brightness is expressed in terms of percent, with 100% denoting the output of the element at 25° C.

The red LED is a semiconductor in layer structure which is composed of four elements (Al, In, Ga, and P). Having a low band gap energy, it decreases in the amount of light emission at a high temperature owing to decrease in the amount of carriers contributing to light emission. Consequently, at about 75° C. for ordinary light emission its luminance decreases to about 70% of that at normal temperature (25° C.). This change is much larger than change in green and blue LEDs.

By contrast, green and blue LEDs composed of three elements (In, Ga, and N) are less vulnerable to temperature change because of their large band gap energy resulting from their shorter wavelengths than red and closer to violet. In fact, it is noticed fromFIG. 15 that the temperature characteristics of the blue LED (B) is almost flat.

The LEDs as the light source for backlight of liquid crystal display units are usually run in such a way that individual LEDs are supplied with a large amount of electric power so as to reduce the total number of LEDs. In addition, the LEDs are connected in series and hence each series greatly fluctuates in load due to heat resistance. As the result, the LEDs fluctuate in heat generation and hence in temperature moment by moment. Consequently, the red LEDs greatly decrease in luminance, whereas the blue LEDs almost remain unchanged in luminance.

Also, the sorrounding air in the display unit is warmed up by the heat generated by LEDs and the relative density of the air decreases. As a result, the air in the display unit rises upwards.

Accordingly, in case which LEDs are used as the light source for backlight of the liquid crystal display units, the liquid crystal display unit with a backlight of LEDs gets hot in the upper part and remains cool in the lower part. The hot upper part decreases in red light output and hence becomes bluish, and the cool lower part remains the same in red light output. As the result, the display unit varies in brightness and tint from its upper part to its lower part.

This problem becomes serious in proportion to the display size.

One way to tackle this problem is to eliminate the variation of display brightness due to temperature distribution.

The gist of the present invention resides in a backlight unit installed in the back of a display unit, which includes a lighting unit composed of a plurality of LEDs connected in series, a drive unit to drive LEDs in the lighting unit, each drive unit corresponding to each lighting unit, and a temperature detecting unit to detect the temperature of each lighting unit, the lighting units being arranged in various positions at which the LEDs keep a prescribed temperature, and the drive units controlling current to be supplied to the LEDs in each lighting unit according to the temperature of each lighting unit, which is detected by the temperature detecting unit, so that each lighting unit keeps brightness uniform even though each lighting unit changes in temperature.

According to one embodiment of the present invention, the LEDs in each lighting unit are arranged in series horizontally and the lighting units are arranged at positions where the temperature is approximately the same because of the temperature distribution resulting from the heat generation of the lighting unit per se. Therefore, the drive unit controls current to be supplied to LEDs in each lighting unit according to the temperature, which is detected by the temperature detecting unit, thereby keeping the backlight unit uniform in brightness even though the lighting unit changes in temperature.

The gist of the present invention resides also in a display unit which includes a non-luminous screen of transmissive-type, a lighting unit which is placed behind the screen and is composed of a plurality of LEDs connected in series, a drive unit to drive LEDs in each lighting unit, the drive unit corresponding to each lighting unit, and a temperature detecting unit to detect the temperature of each lighting unit, the lighting units being arranged in various positions at which the LEDs keep a prescribed temperature, and the drive units controlling current to be supplied to the LEDs in each lighting unit according to the temperature of each lighting unit, which is detected by the temperature detecting unit, so that each lighting unit keeps at least brightness uniform even though each lighting unit changes in temperature.

The backlight unit and display unit according to the present invention employ, as the light source, lighting units each composed of a plurality of LEDs connected in series. The LEDs constituting each lighting unit are arranged in varied positions where they keep a prescribed temperature, and the drive units control current to be supplied to the LEDs in each lighting unit, so that each lighting unit keeps a uniform brightness even though each lighting unit changes in temperature.

The backlight unit and display unit constructed as mentioned above retain a constant brightness and tint irrespective of temperature distribution on the screen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing the structure of the color liquid crystal display unit of backlight type to which the present invention has been applied.

FIG. 2 is a block diagram showing the structure of the drive circuit of the color liquid crystal display unit mentioned above.

FIG. 3 is a schematic diagram showing the arrangement of LEDs in the backlight unit constituting the color liquid crystal display unit mentioned above.

FIG. 4 is a schematic diagram showing the arrangement of LEDs mentioned above in which individual LEDs are represented by diode symbols for circuit diagrams.

FIG. 5 is a schematic diagram showing a unit cell in which six diodes (two each of red, green, and blue diodes) are arranged in one line.

FIG. 6 is a schematic diagram showing threeunit cells4 joined together.

FIG. 7 is a schematic diagram showing the actual connection of LEDs in thelight source21 of the backlight unit.

FIG. 8 is a schematic diagram showing the connection of LEDs in the backlight unit mentioned above.

FIG. 9 is a schematic diagram showing the temperature distribution in the display unit.

FIG. 10 is a schematic diagram in which the temperature distribution in the display unit is superposed on the connection of LEDs in the backlight unit.

FIG. 11 is a diagram which is used to estimate the temperature at each position from one temperature sensor and the temperature distribution pattern.

FIG. 12 is a diagram showing the drive circuit to drive LEDs.

FIGS. 13A to 13C are diagrams showing PWM pulses to drive individual red, green, and blue LEDs.

FIGS. 14A to 14C are diagrams showing signals to drive individual red, green, and blue LEDs, with peak values and PWM controlled.

FIG. 15 is a diagram showing the fundamental temperature characteristics of red, green, and blue LEDs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be described in more detail with reference to the accompanying drawings.

The present invention is applicable to the colorliquid crystal unit100 of backlight type constructed as shown inFIG. 1.

The color liquidcrystal display unit100 consists of acolor display panel10 of transmissive type and abacklight unit20 placed behind thecolor display panel10.

The color liquidcrystal display panel10 of transmissive type has theTFT substrate11 and theopposed electrode substrate12, which face each other with the liquid crystal layer13 (of twisted nematic TN liquid crystal) interposed between them. TheTFT substrate11 hassignal lines14 andscanning lines15 formed thereon in a matrix pattern. At their intersections are formed thin-film transistors16 (as switching elements) andpixel electrodes17. The thin film transistors16 are sequentially selected by thescanning lines15; they also write the image signals (supplied from the signal lines14) into the correspondingpixel electrodes17. On the inside of the opposedelectrode substrate12 are formedopposed electrodes18 and color filters19.

The color liquidcrystal display unit100 has the color liquidcrystal display panel10 of transmissive type mentioned above, which is held between two polarizers and illuminated with white light from its rear by thebacklight unit20. Upon active matrix driving, it produces full-color images as desired.

Thebacklight unit20 has thelight source21 and thewavelength selecting filter22, so that it illuminates (from rear through the wavelength selecting filter22) the colorliquid crystal panel10 with the light emitted from thelight source21.

The color liquidcrystal display unit100 is driven by thedrive circuit200 whose electrical block diagram is shown inFIG. 2.

Thedrive circuit200 consists of the power source110 (to supply the color liquidcrystal display panel10 and thebacklight unit20 with electric power), theX driver circuit120 and the Y driver circuit130 (to drive the color liquid crystal display panel10), the RGB processing unit150 (to be supplied with external image signals through the input terminal140), theimage memory160 and the controller150 (which are connected to the RGB processing unit150), and the backlight control unit180 (to drive and control the backlight unit20).

In thedrive circuit200, the image signals entered through theinput terminal140 are processed such as a chromatic process or the like by theRGB processing unit150 and the processed signals are converted from composite signals into RGB separate signals suitable for the driving of the color liquidcrystal display panel10. The RGB separate signals are supplied to thecontrol unit170 and also to theX driver120 through theimage memory160. Thecontrol unit170 controls theX driver120 andY driver130 at a prescribed timing corresponding to the RGB separate signals. The RGB separate signals supplied to theX driver120 through theimage memory160 drive the color liquidcrystal display panel10, thereby producing images corresponding to the RGB separate signals.

Thebacklight unit20 is placed behind the color liquid crystal display panel oftransmissive type10, so that it illuminates directly the rear of the color liquidcrystal display panel10. Thelight source21 of thebacklight unit20 consists of a plurality of light emitting diodes (LEDs). These LEDs are divided into several groups, which are driven individually.

The LEDS constituting thelight source21 of thebacklight unit20 are arranged in the following manner.

FIG. 3 schematically shows the arrangement of LEDs. Each unit cell (4-1 and4-2) has six LEDs (two each ofred LEDs1,green LEDs2, and blue LEDs3) which are arranged in one line.

In this case mentioned above, theunit4 has six LEDs. However, the number of LEDs for each color in one unit cell is not limited to the one mentioned above; it may be properly varied to produce a well-balanced white color (mixed color) according to the rating and emitting efficiency of LEDs employed.

In the case shown inFIG. 3, the unit cells4-1 and4-2 have the identical arrangement of LEDs, and they are connected to each other at the center (indicated by arrows).FIG. 4 shows the unit cells4-1 and4-2 connected to each other, in which LEDs are represented by symbols for electric circuit drawings. Thered LEDs1,green LEDs2, andblue LEDs3 are connected in series separately, with their polarity conforming to the direction of current flow (left to right).

FIG. 5 shows oneunit cell4 consisting of six LEDs (two each ofred LEDs1,green LEDs2, and blue LEDs3) which are arranged in one line. Each set of two LEDs is represented by2G,2R, and2B as shown inFIG. 5, and the set (2G2R2B) denotes the basic unit as the pattern of six LEDs including two each of green, red, and blue. InFIG. 6, threeunit cells4 joined in series is represented by 3*(2G2R2B) or (6G6R6B).

The LEDs constituting thelight source21 of thebacklight unit20 are connected in the following manner.

The LEDs in thelight source21 are grouped into intermediate units (6G6R6B) which are arranged in a matrix pattern (five horizontal rows and four vertical columns), as shown inFIG. 7. Each intermediate unit consists of three basic units (2G2R2B). Therefore, there are 360 LEDs in total.

These intermediate units (6G6R6B) are electrically connected in the horizontal direction on the screen. The arrangement of intermediate units in this manner results in a plurality of the groups ofLEDs30 connected side by side horizontally formed in thelight source21 of thebacklight unit20 as shown inFIG. 8.

Each group ofLEDs30 connected in series horizontally is provided with the independentLED drive circuit31 in thebacklight unit20. TheLED drive circuit31 supplies current to the groups ofLEDs30, thereby causing them to emit light.

The groups ofLEDs30 horizontally connected in series are arranged such that those LEDs in a certain region have approximately the same temperature when the temperature distribution is gauged in thebacklight unit20.

FIG. 9 shows the temperature distribution on the screen of the color liquidcrystal display unit100, with thebacklight unit20 working. Densely hatched parts indicate the region of high temperature and lightly hatched parts indicate the region of low temperature. It is noted fromFIG. 9 that the color liquidcrystal display unit100 has a high temperature in its upper region and a low temperature in its lower region.

FIG. 10 is schematic diagram in which the temperature distribution in the display unit (which is shown inFIG. 9) is superposed on the connection of LEDs in the backlight unit (which is shown inFIG. 8). It is noticed fromFIG. 10 that connecting LEDs in the horizontal direction is equivalent to connecting LEDs at approximately the same temperature.

As shown inFIG. 10, thebacklight unit20 is provided withtemperature sensors32 to detect the temperature of each group ofLEDs30. There are a plurality oftemperature sensors32 arranged vertically, each corresponding to the group ofLEDs30 horizontally connected in series. The arrangement oftemperature sensors32 may be modified as shown inFIG. 11. The display unit shown inFIG. 11 has only onetemperature sensor32 at its center, and it also has a memory recording the pattern of temperature distribution in the vertical direction. Thetemperature sensor32 can estimate from the detected value of the temperature at different regions across the screen in the vertical direction, by referencing the content of the memory. The information about temperature detected by thetemperature sensor32 is sent to theLED drive circuit31 that drives the corresponding groups ofLEDs30.

In addition, as shown inFIG. 10, thebacklight unit20 is provided the light quantity sensor orchromaticity sensor33, which detects the luminance or chromaticity of each group ofLEDs30. The embodiment shown inFIG. 10 has a plurality of luminance orchromaticity sensors33, each corresponding to the groups ofLEDs30 horizontally connected in series. This embodiment may be modified such that thebacklight unit20 has only one luminance orchromaticity sensor33 if it is provided with a diffuser panel that uniformly and efficiently mixes the colors of individual LEDs. In this case, the luminance or chromaticity value detected by the luminance orchromaticity sensor33 is supplied to theLED drive circuit31 that drives the corresponding groups ofLEDs30. However, it is possible to use only one luminance orchromaticity sensor33 if thebacklight unit20 is provided with a diffuser panel or any other optical system that uniformly and effectively mixes LEDs colors.

TheLED drive circuit31, which is placed in the backlightdrive control unit180, drives the groups ofLEDs30 which are horizontally connected in series in the following manner.

FIG. 12 shows an example of theLED drive circuit31.

TheLED drive circuit31 has the DC-DC converter41, the constant resistance (R_c)42, theFET43, thePWM control circuit44, thecapacitor45, thesample holding FET46, theresistance47, and thehold timing circuit48.

The DC-DC converter41 receives a DC voltage V_INfrom thepower source110 shown inFIG. 2. Then it converts its input (DC power) into a stabilized DC output voltage V_CCby switching. In other words, the DC-DC converter41 generates a stabilized output voltage V_CCin such a way that a predetermined value (V_ref) of potential difference is obtained between the voltage entered through the feedback terminal V_fand the output voltage V_CC.

The anode of the group ofLEDs30 connected in series is connected to the output terminal for output voltage V_CCof the DC-DC converter41 through the constant resistance (R_c). In addition, the anode of the group ofLEDs30 connected in series is also connected to the feedback terminal of the DC-DC converter41 through the source-drain of thesample holding FET46. The cathode of the group ofLEDs30 connected in series is grounded through the source-drain of theFET43.

The gate of theFET43 receives PWM signals from thePWM control circuit44. TheFET43 works in such a way that the source-drain channel becomes on when the PWM signal is on, and the source-drain channel becomes off when the PWM signal is off. Therefore, theFET43 supplies current to the groups ofLEDs30 when the PWM signal is on and shuts off current to the groups ofLEDs30 when PWM signal is off. In other words, theFET43 causes the groups ofLEDs30 to emit light when the PWM signal is on and causes the groups ofLEDs30 to suspend light emission when the PWM signal is off.

ThePWM control circuit44 generates the PWM signal as a binary signal to control the duty ratio of on-time and off-time. ThePWM control circuit44 receives the value of light quantity detected by theluminance sensor33, and it controls the pulse width of PWM signal so that the luminance of the groups ofLEDs30 connected in series coincides with the predetermined value of luminance. In other words, thePWM control circuit44 increases the pulse width, thereby increasing the duration of light emission of the groups ofLEDs30, when the value of light quantity detected by theluminance sensor33 is lower than the predetermined value of luminance. It also reduces the pulse width, thereby reducing the duration of light emission of the groups ofLEDs30, when the value of light quantity detected by theluminance sensor33 is higher than the predetermined value of luminance.

Thecapacitor45 is placed between the output terminal and the feedback terminal of the DC-DC converter41. Theresistance47 is connected to the output terminal of the DC-DC converter41 and the gate of thesample holding FET46.

Thehold timing circuit48 receives the PWM signal and generates the hold signal which is off for a prescribed period of time at the edge of rise of the PWM signal and which is on otherwise.

The gate of thesample holding FET46 receives the hold signal delivered from thehold timing circuit48. Thesample holding FET46 works in such a way that the source-drain becomes on when the hold signal is off and the source-drain becomes off when the hold signal is on.

TheLED drive circuit31 mentioned above supplies current I_LEDto the groups ofLEDs30 only when the PWM signal from thePWM control circuit44 is on. The sample holding circuit is composed of thecapacitor45, thesample holding FET46, and theresistance47. The sample holding circuit samples the value of voltage at the anode of the groups of LEDs30 (that is, at one end of theresistance42 which is not connected to the output voltage V_CC) when the PWM signal is on and then supplies it to the feedback terminal of the DC-DC converter41. The DC-DC converter41 is designed to stabilize the output voltage V_CCaccording to the voltage value entered to the feedback terminal. Therefore, it is possible to make the peak value constant for the current I_LEDthat flows to theconstant resistance R_c42 and the groups ofLEDs30.

Consequently, theLED drive circuit31 performs pulse driving according to the PWM signals while keeping the peak value constant for the current I_LEDflowing into the groups ofLEDs30. Thus, theLED drive circuit31 is so controlled as to make the luminance of light emitted from the groups ofLEDs30 connected in series equal to the predetermined value of luminance according to the amount of light detected by theluminance sensor33.

Moreover, theLED drive circuit31 also has the desiredvalue control circuit49, which is designed to adjust the stabilized voltage value V_reffor the output voltage V_CCof the DC-DC converter41 according to the value of temperature output from thetemperature sensor32.

The desiredvalue control circuit49 receives the value of temperature of the groups ofLEDs30 connected in series which has been detected by thetemperature sensor32. The desiredvalue control circuit49 has the table50 which stores values indicating the change in luminance for change in temperature. While referencing this table50, it generates the desired voltage value V_refso that the groups ofLEDs30 keep a constant luminance even though they change in temperature. The desired voltage value V_refgenerated by the desiredvalue control circuit49 is given to the DC-DC converter41.

The DC-DC converter41 generates the output voltage V_CCso that the desired voltage V_refis obtained for the potential difference between the voltage entered from the feedback terminal V_fand the output voltage V_CC.

TheLED drive circuit31, which is provided with the desiredvalue control circuit49 as mentioned above, adjusts the peak value of the current I_LEDflowing to the groups ofLEDs30 in response to the desired voltage value V_refgenerated by the desiredvalue control circuit49. Thus, the groups ofLEDs30 connected in series emit light at a constant luminance, without change in pulse width of PWM signals, even though they change in temperature.

According to the embodiment mentioned above, the color liquidcrystal display unit100 has the groups of LEDs connected in series under control by the same drive circuit, which are arranged in the region of the same temperature immediately behind the display unit. In addition, the color liquidcrystal display unit100 adjusts current for the groups of LEDs so as to retain a constant luminance even though the temperature changes. To be concrete, it performs PWM control for drive current so that the LEDs emit light with a constant luminance and it also adjusts the peak value of drive current so that the LEDs emit light with a constant quantity of light even though they change in temperature.

Thus, the color liquidcrystal display unit100 is intended to eliminate the fluctuation in luminance on the screen which results from variation in temperature.

TheLED drive circuit31 may be modified as follows.

As mentioned above, thebacklight unit20 is designed such that a certain number of LEDs connected in series are driven as a whole.

LEDs for red (R), green (G), and blue (B) primary colors need different drive circuits because they differ in emission efficiency. In other words, they differ in power consumption and emission efficiency. Therefore, the pulse width adjusted by PWM control varies for different colors. For example, about 50% of on time is enough red LEDs immediately after lighting because red LEDs have high emission efficiency at a low temperature, whereas about 80-90% of on-time is necessary for blue LEDs which are poor in emission efficiency. In other words, different drive circuits are necessary for different colors.

Since the pulse width adjusted by PWM control for each color varies, it follows that the resolution assigned to adjustment of the pulse width for PWM control differs for the luminance of the groups ofLEDs30. For example, the resolution is rough for red and fine for blue. This means that the accuracy of adjustment varies depending on colors. The uneven accuracy of resolution for different colors is a hindrance to uniform white color.

FIGS. 13A to 13C show the difference in resolution in PWM control for different colors.

InFIG. 13A to 13C, it is assumed that the PWM control pulse can be divided in 256 steps (8 bits) to adjust a pulse width.FIG. 13A shows the PWM control pulse for red;FIG. 13B shows the PWM control pulse for green, andFIG. 13C shows the PWM control pulse for blue. It is assumed that it is necessary to mix the red, green, and blue colors in the following ratio to obtain the desired white color.

• Blue with a PWM width of 256 (100% duty)
• Green with a PWM width of 191 (75% duty)
• Red with a PWM width of 126 (50% duty)
  The blue color can be adjusted in 256 steps, whereas the red color can be adjusted only in 126 steps (or 7 bits). In addition, one-step width for blue corresponds to two-step width for red color. It is possible to improve the resolution by increasing the number of bits; however, this approach is not practical because it needs an expensive converter with a high accuracy.

For the reasons mentioned above, theLED drive circuit31 changes the peak value of current (supplied to the groups of LEDs30) so that the resolution of pulse width is the same for red, green, and blue LEDs. This is accomplished in practice by entering the control value of PWM into the desiredvalue control circuit49 to adjust the desired voltage value V_refat which the output voltage V_CCremains stable.

If it is assumed that when the mixing ratio of red, green, and blue colors is adjusted for the same peak value of current, the PWM width for blue is 256 (100% duty), the PWM width for green is 191 (75% duty), and the PWM width for red is 126 (50% duty), then the peak value of current for red color should be adjusted to 50% as shown inFIG. 14A, the peak value of current for green color should be adjusted to 75% as shown inFIG. 14B, and the peak value of current for blue color should be adjusted to 75% as shown inFIG. 14C.

By changing the peak value of current for the groups ofLEDs30, it is possible to have the same resolution for red, blue, and green at the time of adjustment and hence it is possible to keep a balance for the accuracy of control on different colors.

Claims (11)

1. A backlight unit installed in the back of a display unit, comprising:

lighting units, each lighting unit comprising:

red, green, and blue light emitting diodes connected in series, and

a temperature detecting unit to detect the temperature of the lighting unit, and

a drive unit to control current supplied to the light emitting diodes according to the temperature of the lighting unit, so that each lighting unit keeps a uniform brightness even though each lighting unit changes in temperature, to adjust current to be supplied to the light emitting diodes by means of pulse width modulation, so as to keep at least the brightness of light emitted from each lighting unit uniform, to control the peak value of the current to be supplied to the light emitting diodes, so that the resolution of pulse width is the same for the red, green, and blue light emitting diodes, and to adjust the peak value of current to be supplied to the light emitting diodes in response to the detected temperature,

wherein the lighting units are arranged in various positions at which the light emitting diodes keep a prescribed temperature.

2. The backlight unit as defined inclaim 1, wherein the drive unit controls current supplied to the light emitting diodes, so that each lighting unit keeps a uniform brightness as well as a uniform chromaticity even though each lighting unit changes in temperature.

3. The backlight unit as defined inclaim 1, wherein the lighting units are placed behind the display unit such that the light emitting diodes are connected in series and arranged in the horizontal direction of the screen.

4. The backlight unit as defined inclaim 1, wherein the temperature detecting unit detects the temperature of one of the lighting units having a malfunctioning temperature sensor according to information from a memory which previously stored the temperature of the lighting unit having the malfunctioning temperature sensor in response to the temperature of one of the lighting units at the position where a temperature sensor is installed.

5. The backlight unit as defined inclaim 1, further comprising a light detecting sensor to detect the brightness of light emitted from the light emitting diodes in each lighting unit, wherein the drive unit adjusts current supplied to the light emitting diodes so as to keep at least the brightness of light emitted from each lighting unit uniform.

6. A display unit comprising:

a non-luminous screen of transmissive-type,

lighting units, placed behind the screen, each lighting unit comprising:

red, green, and light emitting diodes connected in series, and

a temperature detecting unit to detect the temperature of the lighting unit, and

a drive unit to control current supplied to the light emitting diodes according to the temperature of the lighting unit, so that each lighting unit keeps a uniform brightness even though each lighting unit changes in temperature, to adjust current to be supplied to the light emitting diodes by means of pulse width modulation, so as to keep at least the brightness of light emitted from each lighting unit uniform, to control the peak value of the current to be supplied to the light emitting diodes, so that the resolution of pulse width is the same for the red, green, and blue light emitting diodes, and to adjust the peak value of current to be supplied to the light emitting diodes in response to the detected temperature

wherein the lighting units are arranged in various positions at which the light emitting diodes keep a prescribed temperature.

7. The display unit as defined inclaim 6, wherein the drive unit controls current supplied to the light emitting diodes, so that each lighting unit keeps a uniform brightness as well as a uniform chromaticity even though each lighting unit changes in temperature.

8. The display unit as defined inclaim 6, wherein the lighting units are placed behind the display unit such that the light emitting diodes are connected in series and arranged in the horizontal direction of the screen.

9. The display unit as defined inclaim 6, wherein the temperature detecting unit detects the temperature of one of the lighting units having a malfunctioning temperature sensor according to information from a memory which previously stored the temperature of the lighting unit having the malfunctioning temperature sensor in response to the temperature of one of the lighting units at the position where temperature sensor is installed.

10. The display unit as defined inclaim 6, further comprising a light detecting sensor to detect at least the brightness of light emitted from the light emitting diodes in each lighting unit, wherein the drive unit adjusts current to be supplied to the light emitting diodes so as to keep at least the brightness of light emitted from each lighting unit uniform.

11. The display unit as defined inclaim 6, which further comprises an optical system that uniformly mixes the colors of light emitted from each light emitting diode in each lighting unit, and wherein the drive unit acquires as a representative value the brightness or chromaticity detected by the light detecting sensor and adjusts current to be supplied to each light emitting diode in each lighting unit so that the lighting unit changes uniformly as a whole and the light emitted from each lighting unit remains constant in brightness or chromaticity.

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